Method for post-tensioning tendons

ABSTRACT

An anchoring system is provided wherein the end of a posttensioned tendon extending from a concrete structure is encased in a metallic sheath and the sheath is crimped in a series of reversals of increasing angularity in the direction away from the concrete structure.

United States Patent inventor Appl. No. Filed Patented Assignee FredericA. Lang Landenberg, Pa. 858,948

Sept. 18, 1969 Division of Ser. No. 712,796, Mar. 13, 1968, Pat. No.3,513,609. Continuation-in-part of application Ser. No. 623,709, Mar.16, 1967, now abandoned.

May 25, 1971 E. l. du Pont De Nemours and Company Wilmington, Del.

METHOD FOR POST-TENSIONING TENDONS 5 Claims, 36 Drawing Figs.

U.S. Cl

Int. Cl

Primary Examiner-Frank L. Abbott Assistant Examiner.lames L. Ridgill,Jr. Attorney-George W. Walker ABSTRACT: An anchoring system is providedwherein the end of a post-tensioned tendon extending from a concretestructure is encased in a metallic sheath and the sheath is crimped in aseries of reversals of increasing angularity in the direction away fromthe concrete structure.

a t 1o 4 A I PATENTED M25 1911 SHEET 1 BF 8 I. u f

PATENTEU HAYZS 19m SHEET 8 OF 8 METli-IGD FGR POSiT-TENSKONING TENDONSThis application is a division of application Ser. No. 712,796, filedMar. 13, 1968 granted May 26, 1970 as Pat. No. 3,513,609 by the sameinventor, which in turn is a continuation-impart of application Ser. No.623,709, filed Mar. 16, 1967 now abandoned, by the same inventor.

This invention relates to concrete structures in which the concrete ismaintained in compression during use so as to prevent failure intension, and to tendons and anchoring systems for maintaining thisconcrete under compression.

In the post-tensioning method of prestressing concrete, concrete ispoured into a form which defines the shape of the structure desired andwhich contains tendons extending across the form. After the concrete hassufiiciently hardened, the tendons are elongated to a point within theirelastic limit, and their ends are held by fasteners that rest againstthe concrete or against a bearing plate which rests against theconcrete. In this manner, the tensile force in the tendons produce acompressive force in the concrete and, as a result, increases the loadcarrying capacity of the concrete.

Important to the success of post-tensioning is the requirement that thetendon be free to move within the concrete during post-tensioning sothat the elongation is as evenly distributed as possible along thelength of the tendon. One approach for accomplishing this result hasbeen to move the tendon back and forth in the concrete during itshardening to make a chase (U.S. Pat. No. 3,029,490 to Middendorf).Another approach has been to fonn the tendon from a greasecoated bundleof rods which is then helically wrapped with plastic or paper tape andembedded in the concrete. After the concrete hardens around the tape,some movement between the rods and the tape can occur duringpost-tensioning. The same principle is employed for a tendon composed orrods or cable which has been threaded through a metal or plastic tubing.Special tubing has been designed for this purpose (U.S. Pat. No.2,677,957 to Upson and U.S. Pat. No. 3,212,222 to Wittfoht). Theseapproaches have one or more disadvantages, e.g., the presence ofrelatively high friction which causes nonuniform elongation of thetendon during post-tensioning, the presence of measurable voids (betweenthe tube or tape and the rod or cable) which expose the tendon tocorrosion, and the requirement for tendons of relatively large crosssection which are difficult to transport and handle in the field. Whileprecautions are taken to minimize some of these disadvantages, e.g.,grouting as shown in U.S. Pat. No. 3,114,987 and U.S. Pat. No. 3,060,640to Harris, these precautions require extra expense and still the otherdisadvantages remain.

The present invention provides a tendon which overcomes thedisadvantages of previous tendons. One embodiment of the tendon of thisinvention comprises a wire and plastic material tightly and uniformlycoating said wire. In this embodiment, the plastic material should besuch that it provides a coefficient of friction between the coating ofplastic material and wire of not greater than 0.09. In anotherembodiment, a lubricant is present at the interface between'the wore anduniform tight coating of plastic material to produce the coefficient offriction of not greater than 0.09. The aspects of low friction and tightcoating in these embodiments appear anomalous in that one aspect wouldbe expected to defeat the other. This does not occur, however, in thepresent invention wherein the wire is a single wire of small diametergenerally no greater than 0.3 inch and has a smooth, regular surface andwherein the coating closely conforms to this surface and has a uniformthickness generally no greater than about 0.05 inch. The low coefficientof friction enables the elongation of wire in post-tensioning to besubstantially uniform so that it is more efficiently used than tendonsystems having a higher coefficient of friction. The coating issufficiently tight, however, that the wire cannot be thfeaded into thecoating for any useful distance, Instead,the coating is molded about thewire from the molten state. This tightness precludes voids and thusmarkedly reduces coriosive elements, e.g., air and moisture fromreaching the wire. This, together with additional corrosion protectionif necessary, which may be provided by suitable coating on the wire,enables wire of small diameter and high tensile strength to be safelyused for post-tensioning.

The tendons of this invention overcome the transportation and handlingawkwardness of a relatively heavy gauge tendon in that the tendons ofthis invention can be supplied on a reel of convenient diameter. Noadvance prefabrication of the tendon, such as outfitting with endfastening hardware, is necessary. The use of separate tubing throughwhich rod or cable is threaded is not required. Instead, the tendon ofthis invention is a prepackaged post-tensioning assembly. The tendon ofthis invention is merely unreeled within and across the form for theconcrete, laid in position, severed to appropriate length and thenoutfitted with appropriate end fastening hardware. No special steps arenecessary during the hardening of the concrete. When the concrete hashardened, the plastic coating is stationarily embedded therein. However,the wire is free to move and therefore can be uniformly post-tensioned.

In another embodiment, a plurality of the foregoing described tendons ofthis invention are integrated in the form of a ribbon, with theindividual tendons being in a common plane and being severable from oneanother. This composite tendon has the same advantages of the individualtendon of this invention and additionally the advantage of providingsimultaneous handling for'a plurality of tendons.

Another embodiment of this invention relates to anchoring systems forpost-tensionable tendons. Various anchoring systems for the ends ofpost-tensionable tendons are known, such as disclosed in U.S. Pat. Nos.2,371,882 and 2,270,240, both to Freyssinet; US Pat. No. 2,609,586 toParry; U.S. Pat. No. 2,728,978 to Birkenmaier et al., and U.S. Pat. No.3,216,162 to Gerber et al. These systems suffer from one or more of thedisadvantages of requiring prefabrication of the tendon or the anchoringsystem before reaching the construction site, thereby reducingflexibility of application and increasing expense, of causing excessivelocalized stresses in the post-tensioned tendon, of requiringmultiplicity of parts, of holding by friction rather than by positiveengagement, and/or of failing to provide corrosion protection to theanchored end of the tendon. The present invention provides an anchoringsystem for the ends of post-tensionable tendons which alleviates orovercomes these disadvantages. The anchoring system comprises acrimpable metallic sheath for encasing a protruding end of the tendon,with the sheath having one end in force transmitting relationship withthe concrete structure being placed under compression by thepost-tensioning operation and with the body of the sheath being deformedin a series of reversals of increasing angularity in the direction awayfrom the concrete structure to engage positively the protruding tendonto prevent its withdrawal from the sheath. This maintains the tendonunder tension and thereby, the concrete structure under compression.

In another embodiment of anchoring system, anchoring of the post-tensiontendon is accomplished by bonding of the wire along its length to thecoating of plastic material embedded in the concrete structure. Thisbonding is effected by a curable plastic material present between thewire and the coating, which is cured after the tendon is in place withinthe concrete and post-tensioned.

These and other embodiments will be described more fully hereinafterwith reference to the drawings, in which:

FIG. 1 shows a plan view of a concrete structure incorporating featuresof this invention;

FIG. 2 shows a cross section taken along line 2-2 of the structure ofFIG. 1;

FIG. 3 shows in indeterminate length one embodiment of tendon of thisinvention with a section of its coating removed;

FIG. 4 shows a cross section taken along line 4-4 of the tendon of FIG.3;

FIG. 5 shows a cross section of another embodiment of tendon of thisinvention;

FIG. 6 shows a plan view of a length of composite tendon of thisinvention with the wire component partly exposed;

FIG. 7 shows a side view of the composite tendon of FIG. 6;

FIG. 8 shows a cross section taken along line 8-8 of the compositetendon of FIG. 7;

FIG. 9 to 11 show in cross section other embodiments of compositetendon;

FIG. 12 shows in plan view one form of concrete structure post-tensionedwith a particular pattern of tendons of this invention',

FIG. 13 shows a partially cut away plan view of a concrete structuresimilar to that of FIG. 12 employing a different posttensioned tendonpattern;

FIG. 14 shows in indeterminate length and plan view another form ofconcrete structure post-tensioned with still another pattern of tendonsof this invention;

FIG. 15 shows an enlarged partially cutaway plan view of one end of theconcrete structure of FIG. 14 with the pattern of tendons in greaterdetail;

FIG. 16 shows in still further enlargement a cutaway plan view of aregion of the concrete structure of FIG. 15 showing a reversal of atendon therein;

FIG. 17 shows in plan view a concrete structure posttensioned with stillanother pattern of tendons of this invention;

FIG. 18 shows in plan view one end of the concrete structure of FIG. 17with a variation of the pattern of tendon therein,

FIG. 19 shows in plan view a cantilever concrete construction utilizinga tendon of the present invention;

FIG. 20 shows a cross section of the cantilever concrete constructiontaken along line 20 -20 of FIG. 19;

FIG. 21 shows in indeterminate length a pipe post-tensioned with atendon of this invention;

FIG. 22 shows a cross section of the pipe taken along line 22-22 of FIG.21;

FIG. 23 shows in fragmentary cross section one embodiment for anchoringthe ends of tendons of this invention;

FIG. 24 to 26 show schematically the steps for obtaining the anchoringdepicted in FIG. 23;

FIG. 27 shows schematically a plan view of apparatus suitable foranchoring a post-tensioned tendon in a concrete structure;

FIG. 28 shows a side elevation of the apparatus and end of the concretestructure of FIG. 27;

FIG, 29 shows an enlarged fragmentary plan view of a series of crimpingelements of a jaw suitable for use in the apparatus of FIG. 27;

FIG. 30 shows a still further enlarged plan view of one of the crimpingelements of the jaw of FIG. 29;

FIG. 31 shows a side elevation of the crimping element of FIG. 30;

FIG. 32 shows in fragmentary cross section one end of a concretestructure incorporating another embodiment of anchoring by endfastening, of this invention;

FIG. 33 shows the extremity of the anchored assembly of FIG. 32 afterradial compression for sealing the extremity from moisture;

FIG. 34 shows in fragmentary cross section one end of a concretestructure incorporating still another embodiment of end fastening systemin which the tendon is maintained under tension by external means;

FIG. 35 shows the embodiment of FIG. 34 after crimping and after releaseof the tendon by the external means; and

FIG. 36 shows in enlarged cross section an embodiment of bonded tendonof this invention.

LOW FRICTION TENDON Referring now to the drawings, FIG. 1 shows aconcrete slab 2 which is held under compression by substantiallyuniformly elongated post-tensioned tendons 4 of the present inventionarranged in a conventional crisscross pattern, with the ends of thetendons being secured by end clamps 6 which can include bearing plates.The tendons 4 are spaced from one another at a distance which gives thedesired amount and distribution of compression to the slab 2, which canbe in the form of pavement, floor, wall, roof, siding, i.e., shingles,and tabletops. The

tendons 4 can be in a draped disposition as they extend through the slab2, as shown for one tendon in FIG. 2.

The tendon 4, as shown in FIG. 3, comprises a wire 8, which is elongatedduring post-tensioning, tightly and uniformly coated with plasticmaterial 10, which remains fixedly embedded in the concrete duringpost-tensioning. The wire has a smooth outer surface, and the coating 10has its inner surface in close conformation with the outer surface ofthe wire as shown in FIG. 4. Thus, there are no irregularities betweenthe coating and the wire to interfere with the elongation of the latter.

The diameter of the wire is generally no greater than 0.3 inch andusually no greater than 0.2 inch. The smallest diameter wire that can beused will primarily depend on the size of the concrete shape beingpost-tensioned; for example, music wire measuring about 0.02 inch indiameter is useful in posttensioning small concrete shaped, for example,which are of tabletop size. Generally however, the diameter of the wirewill be at least 0.05 inch. With respect to coating 10, it is continuousand of uniform thickness. Generally no greater than 0.05- inch thickcoating is required. However, the coating thickness usually need be nogreater than 0.015 inch. As a general rule, the coating thickness is nogreater than 25 percent and preferably no greater than 12 percent of thediameter of the wire. The thinness of coating 10 about wire 8 aids inmaintaining the integrity of the coating under transverse loading byminimizing the tendency of the coating to cold flow, possibly exposingthe wire. The tightness of the coating about the wire minimizescorrosive elements such as air and moisture from contacting the wire.

In order to attain the low coefficient of friction between the wire andthe coating, their mating surfaces should be smooth, even and uniformwith respect to each other. Some plastic materials, e.g.,tetrafluoroethylene polymers, i.e., homopolymers and copolymers thereofwith small proportions of other copolymerizable monomers, preferablyperfluorocarbon monomers, generally provide a coefficient of friction of0.09 or less. For other plastic materials it may be necessary to providea lubricant at the interface between the wire and the coating. Such alubricant should be noncorrosive to the wire and should have sufficientfilrn" strength to prevent direct contact between the wire and thecoating of plastic material. The magnitude of the film strength requiredwill depend on the amount of transverse stress placed on the tendonduring post-tensioning. Coefficients of friction of no greater than 0.05are attainable for tendons of the present invention. Such tendons arepanicularly useful in reversal patterns within the concrete. Forexample, for an amount of reversal (total angle of curvature) of about acoefficient of friction of no greater than 0.05 between the wire and thecoating is desirable. As the amount of reversal increases, even lowercoefficients are desired. Coefficients of friction decreasing from 0.02to 0.009 are desirable for angles of curvature increasing from about 540to l300. However, coefficients of friction of 0.09 to 0.03 are generallyuseful in this range as well as at smaller angles of curvature. Theelastic limit of the wire is not exceeded in bending the wire to formany of the reversal patterns herein described.

Lubrication can be accomplished by the incorporation of slip-producingagents in the coating of plastic material, such agents includinggraphite, molybdenum disulfide and oils. A particularly advantageouscoating is polyamide blended with from about I to 5 percent siliconeoil, with the polyamide preferably being 610 nylon. In anotherembodiment, the lubricant can be disposed as a separate layer 12 betweenthe outer surface of the wire 8 and the inner surface of the coating 10,as shown in FIG. 5. Preferably, the lubricant of layer 12 has at leastsome solid lubricating material present therein, particularly on theoccasion when rlie tendon is to be used for reversal patterns.Representative solid and liquid lubricants for layer 12 include themineral oils, hydrocarbon oils, graphites, fluorocarbon polymers such ashigh molecular weight polymers or telomers derived principally fromtetrafluoroethylene used alone or in combination. A particularly usefullubricant layer 12 is composed of a solid fluorocarbon coating such asobtained with the polytetrafluoroethylene dispersions disclosed in U.S.Pat. No. 2,612,484. Preferably, a corrosion inhibitor is present betweenthe wire 8 and coating consisting (i.e., in layer 12) e.g., of achemical reducing agent such as sodium nitrite or high molecular weightorganic amines such as lauryl amine. Along with the corrosion inhibitor,a dispersing agent is preferably also present, such being selected froma wide variety of commercially available water soluble emulsifyingagents, such as inorganic salts of sulfonated petroleum or sulfonatedhydrocarbons, the aryl sulfonamides and high molecular weight fattyacids, i.e., aliphatic monocarboxylic acids or water soluble saltsthereof, especially fatty acids having from 12 to 20 carbon atoms. Thecorrosion inhibitor and dispersion agent are conveniently applied in theform of an emulsion of hydrocarbon oil in water. Most of the water isdriven off during the application of coating 10. A particularly usefulcorrosion inhibitor-lubricant which is useful for applying to the wire 8prior to coating is the aqueous dispersion of 45-47 percent water, l92lpercent hydrocarbon oil, e.g., mineral oil, -16 percent amine, e.g.,ethanolamine, 89 percent ofa fatty acid, e.g, oleic and 7.5 10 percentof sodium nitrite (percents are by weight). Most of the water of thisdispersion is driven off during the application of the coating 10 andthe residue mixture provides both lubrication and corrosion protection.This dispersion can be used in combination with other lubricantshereinbefore described. Mixtures of this dispersion withpolytetrafluoroethylene dispersions provide a layer 12 with bothlubrication and corrosion protection properties. Generally the benefitsof either dispersion become apparent when as little as 10 percent byweight of either dispersion is present in the combination thereof. Thelubricant layer 12 can also include a first coating of lubricous plasticmaterial such as polytetrafluoroethylene applied directly to the wire,with lubricant of the layer 12 being provided between the first coatingand coating 10, whereby the first coating is elongated with the wirewhile the coating 10 performs the same function as hereinbeforedescribed. Liquid lubricants, such as the corrosion inhibitorlubricantdescribed can be present between the first coating and coating 10, as apart oflayer 112.

The layer 12 of lubricant, when present is generally no greater thanabout 0.012 inch in thickness, usually no greater than 0.005 inch, andis uniform along the length of the wire. Preferably, the thickness oflayer 12 does not exceed 0.002 inch. The tight coating 10 of plasticmaterial maintains the lubricant at the wire-plastic coating interfaceduring handling and in post-tensioned use.

The plastic material from which coating 10 is made can consistessentially of any plastic resin which can be formed into a coatingabout the wire and which is compatible with the particular lubricantpresent, if any. The plastic material can be applied as a uniform andtight coating to the wire by extrusion of the plastic material in theform of continuous tubing around a wire which is moving in the directionof the extruded tubing and by drawing the continuous tubing down byvacuum onto the moving wire. When the wire is to be coated withlubricant and/or corrosion inhibitor, this can be done, such as bydipping or spraying or otherwise applying the lubricant onto the wireprior to the extrusion coating step with the extrusion coating hereindescribed not removing the previously applied lubricant or corrosioninhibitor coating from the surface of the wire. This kind of extrusioncoating can be carried out with what is generally referred to as atubing die, such as is described in Paper No. 603 entitled Wire Jacketsof Nylon" by E. C. McKannan arid R. E. Shaw published by the E. 1. duPont de Nemours and Company. When the layer of lubricant has sufficientintegrity prior to the coating with plastic material step, apressure-type extrusion die can be used to fonn the coating 10. Suitableplastics include the polyamides, especially hexamethylene adipamide (66nylon), polycaprolactam (6 nylon) and hexamethylenc subaceamide (610nylon) and copolymers and terpolymers thereof, polyolefins including thealpha-monoolefins having from 2 to 10 carbon atoms, especiallypolyethylene and polypropylene, the polystyrenes, the ABS resins,ionomers such as described in Canadian Pats. 674,595 and 713,631 both toRees, halogenated polyolefins such as vinyl chloride polymer,oxymethylene polymer and copolymer and polyethylene terephthalate.

The wire used in the tendons of this invention, will generally be anyfilamentary material that has an elastic limit (stress) of at least150,000 psi. The wire may be of metal such as cold drawn carbon steel orof fiber glass. The wire can include a surface treatment which improvescorrosion resistance but does not significantly impair strength; anexample of such a surface treatment is a coating of zinc powder whichcan be held in place by a phosphate binder. A particularly useful tendonof this invention is one in which the wire is the carbon steel andmeasures 0.08 inch in diameter, which is coated with a lubricant oilcontaining tetrafluoroethylene polymer particulate solids dispersedtherein to a thickness of about 0.0005 inch, which is in turn tightlyand uniformly coated with 610 nylon or olefin polymer having a thicknessof 0.01 inch.

In another embodiment of tendons of this invention, a composite tendon14 can be made of a plurality of wires 16, which are similar to wires 8,covered by a uniform and tight coating 18 of plastic material, which issimilar to coating 10 hereinbefore described. As shown in FIGS. 6 and 7the wires are disposed in a common plane whereby the composite tendon 14is in the shape of a ribbon. The individual wires 16 of the tendon 14are integrally connected by a relatively thin bridge 20 of the sameplastic material as the coating 18. This tendon can be applied inconcrete construction from a reel in the same manner as tendon 4 withthe additional advantage of simultaneous handling of wires 16 beingpresent.

Should it be desired toseparate one coated wire 16 from another suchwire of the tendon 14, either for the purpose of providing individualendsof the tendon for purposes of end fastening, the junction betweenthe coating 18 of each of the wires can be in the form of a sharpV-shape 22 as shown in FIG. 9. In another embodiment, as shown in FIG.10, the bridge 20 may be in the form of a web 24 between coated wires 16with opposing V-shape grooves 26 disecting the webs 24 to assist inseparation of one coated wire 16 from the other.

In still another embodiment of composite tendon in ribbon form, aplurality of tendons shown in FIG. 4 can be coated with an outer coatingof plastic material 28 which forms the junction between the individualtendons 4. The configuration of the coating 28 at the junction of thetendons 4 can be varied as desired such as to resemble the junctionsdepicted for the embodiments of FIG. 8 to 10. I

The composite tendons of this invention can be made of the samematerials and have the same frictional characteristics and coatings 12as discussed hereinbefore with respect to tendon 4.

In addition to the crisscross application of tendons of this inventionas depicted in FIG. 1, tendons of this invention may also be used forunidirectional post-tensioning of concrete structures, particularlythose which have much greater length than width. For example, aplurality of tendons 4 can be used to post-tension such a concretestructure 30 which can be in the shape of a railroad tie. Tendons 4 areheld at their ends by end clamps 6 as shown in FIG. 12.

Because of the light gauge and low frictional characteristics of thetendons of this invention, a similar elongated concrete structure 32 canbe post-tensioned by a tendon 4 which is disposed in a loop patternwithin the structure and which requires the use of end clamps at onlyone end thereof. In this embodiment the end clamps are situated within acavity 34 in one end of the concrete structure 32 which can also be usedas a railroad tie. 1n this embodiment, it is apparent that only half asmany ends of the tendon 4 need to be end-clamped as compared to theembodiment of FIG. 12. The point at which the looped tendon 4 of FIG. 13can be terminated can be varied to the location desired. For example,one or both ends of the tendon 4 can be made to terminate in a cavity(not shown) which is intermediate the ends of the concrete structure 32.

Another tendon pattern which is useful for post-tensioning concretestructures having greater length than width is the zigzag pattern oftendon 4 in an elongated concrete structure 36 (FIG. 14) which isuseful, for example, as a highway or airport runway slab. Such a tendoncan be positioned within the form for the slab by a concrete pavingmachine just prior to filling of the form with concrete. By having areel of the tendon move transversely in response to longitudinalmovement of the paving machine, the zigzag pattern is obtained. In thiszigzag pattern, tensioning of the tendon 4 is done at opposite ends ofthe concrete structure 36, but the tendon essentially always travels atan angle first to one side 38 and then to the other side 40 and so on,reversing its direction at a point proximate to the sides as the tendonpasses from end to end of the structure 36. Upon post-tensioning of thetendon 4, the structure 36 is placed under both longitudinal andtransverse compression. In practice, a plurality of the tendons 4 areused forming reversals spaced along the length of the structure and inopposing sequence, i.e., opposing reversals at the sides 38 and 40 asshown in FIG. 15.

Pins 42 may be disposed in the concrete form in which the particularconcrete structure is to be made and the tendon, such as tendon 4, ispassed around such pin prior to pouring of the concrete in the form. Thepin 42 has a sufficiently large radius that the elastic limit of thewire is-not exceeded in bending (reversing) about the pin. The pin canremain buried in the concrete structure, such as shown in FIG. 16. Thispin or similar device can be used in any of the reversal tendon patternsdescribed herein. Another pattern which can be produced with tendons ofthe present invention is similar to that of FIGS. 14 and 15, except thatthe ends of the tendons all protrude from, and are thus clamped at, oneend of the concrete. This embodiment is shown in FIG. 17, wherein onetendon 44 extends in one direction in a zigzag path within concretestructure 46 and then at the end thereof, reverses itself to form asimilar return path and provide side-by-side ends for end clamping withclamps 6. Another tendon 48 extends in an opposing course through thestructure 46 to also have its ends terminate side-by-side as shown. In avariation on the tendon pattern in FIG. 17, a single tendon 50 can beused to establish essentially the same pattern as shown in FIG. 17 forconcrete structure 46, by uniting the pattern with segment 52 of thesingle tendon, whereby only two ends require post-tensioning andfastening with end clamps 6. Tendons 44, 48 and 50 are similar to tendon4. In practice, a plurality of tendons, having their ends and reversalpoints spaced from that of other tendons, will be used to form thetendon pattern of FIGS. 14 and 15.

Another pattern which can be formed of tendons of this invention is apattern of circumferentially displaced radially-extending loops ofeither one or a multiplicity of tendons. An example of this pattern isshown in FIG. 19 and FIG. in which a deck or flooring 54 extends incantilever fashion from a central tubular section 56, which in turn issupported by a footing 58 which is buried in the ground 60 or isotherwise supported. The deck 54 is circular in plan view and ispositioned symmetrically about the tubular section 56. Embedded in thedeck 54 is a tendon 4 entering from the interior 62 of the tubularsection 56 and exiting within the same tubular section 56 and exitingwithin the same tubular section and secured therein by end clamps 6.Within the deck 54, the tendon 4 forms a series of substantiallyradially extending circumferentially displaced loops uniformly generatedabout the tubular section 56. The

ends oftendon 4 extend through apertures in a steel ring 63. Uponpost-tensioning, the deck 54 is compressed inwardly, with the steel ringserving to transmit the tensile force in one end of the tendon to theother end thereof. End clamps 6 maintain the tendon in thepost-tensioned condition. In this pattern as in previous patterns, aplurality of tendons spaced one from the other, will be used. I

Tendons of this invention can also be used for post-tensioning concretepipe and the like by disposing the tendon, e.g., tendon 4, in a helicalpath within the wall ofa pipe 66, held in the tensioned conditioned byend clamps 6 as shown in FIGS. 21 and 22. Although only one tendon isshown, normally a plurality of such tendons will be used with the endsterminating at spaced points or about the outer surface of the pipe asshown for tendon 4 or about the ends 69 of the pipe. The helix angle ofeach tendon will depend on the angle of wrap desired for the particulartendon used. For low coefficients of friction, e.g., 0.009, a relativelylarge angle of wrap can be employed which means for a given length ofpipe, the helix angle of the tendon can be less than for tendons havinga higher coefficient of friction. Each tendon need not go fromend-to-end of the pipe but can terminate at an intermediate point, withother helically disposed tendons starting progressively later andextending progressively further along the length of the pipe. Thepost-tensioned concrete pipe is made, for example, by arranging a wiremesh cylinder within the form for the pipe and con centrically about itslongitudinal axis, helically winding the number of tendons desired aboutthis cylinder, pouring the concrete in the form, allowing the concreteto harden sufficiently, followed by post-tensioning and end clamping thetensioned wires. Preferably the tendons within the wall of the pipe 66are closer to the outer surface 67 than the inner surface 68 thereof.

Composite tendons such as shown in FIGS. 6 through 11 can be used ineach of the patterns depicted herein. For patterns entailing a reversalin the direction of travel of the tendon, the plane of the compositetendon need only be made parallel to the pin 42 at the point of reversaland wound therearound similar to the manner shown for tendon 4 in FIG.16.

As is evident from the tendon patterns of FIGS. 13, I4, 17, 18 and 19,the tendons of this invention can be used in patterns requiring extremereversal conditions. For example, when the total angle of wrap for atendon having a coefficient of friction of about 0.009 or as high as0.02 is 180, the variation of stress along the length of the wire isbarely noticeable. Patterns in which the angle of wrap is 1,000 or morecan be employed in which the stress at one end of the tendon is withinat least 70 percent, preferably at least percent, of the stress at theopposite end. The path of the tendon from reversal point to reversalpoint can be substantially straight; however, the path can be draped aswell. By tensioning the wire at both ends instead of just one end, theangle of wrap (curvature) is reduced by one-half (the example angles ofcurvature hereinbefore recited and the coefficients of frictiondesirable thereat refer to tensioning at one end). Thus, for the patternof FIG. 19, while the tendon 4 undergoes sufficient reversals to totalabout 2,000 for the pattern, each end of the tendon need only betensioned around about 1.000 angle of wrap. Preferably, however, thepattern of FIG. 19 is made with two tendons so as to provide four endsfor tensioning; this can be done by interrupting the single tendon shownin FIG. 19 at or near its midpoint and having the resultant ends extendthrough the steel ring 63. The consistency of stress and thus uniformityof elongation can be determined experimentally using the belt formula,which is F =F e wherein F =jacking end tension, F tension at other endof wire and p.= coefficient of friction and theta is the total angle ofcurvature or wrap of tendon, and e is the natural logarithm base. Thecoefficients of friction recited herein are determined by using thisformula. For further explanation of the formula, reference is made to T.Y. Lin, Design of Prestressed Concrete Structures, Second Edition, 1963,page 110.

Any of the anchoring systems of the prior art which is effective onsmall diameter tendons may be used to anchor tendons of this invention.A suitable end clamp is shown, for example, in US. Pat. No. 3,137,971 toRhodes.

9 ANCHORING SYSTEMS Another anchoring system which may be used, butwhich use is not limited to the tendons of this invention, is that whichis shown in FIG. 23. In this FIG. is shown a portion of a concretestructure 70 terminating in an outer surface 72 which has a cavity 74therein. .Protruding from the recessed surface 76 of the cavity is atendon which for illustrative purposes is an end of a tendon 4 in theelongated condition but could be the end of any known tendon or anindividual tendon of the composite tendon or an individual tendon of thecomposite tendon of FIG. 6. The protruding portion of tendon 4 iscrimped in a series of reversals which increase in angularity, that is,a progressive increase in the size of angles A, B, C and D in that order(the bends in the tendon become sharper) in the direction away from thesurface 76. Running coextensively with crimped portion of the tendon 41is a tubular metallic sheath 78. The sheath 78 confines the crimp of thetendon 4 to prevent it from withdrawing into the concrete structure 70and relieving the compression on the concrete. The degree of bending inthe crimp is sufficiently high so that the axial stress required towithdraw the crimped tendon into the concrete member 70 at all times isgreater than the operating stress on the tendon (wire 8), generally atleast 30 percent greater; therefore, this withdrawal does not occur.Generally, the bend angle of the reversals is not sufficient to destroythe continuity of the plastic jacketting, if such jacketting is present,thereby maintaining its integrity for corrosion protection purposes. Thecompressive force on sheath 70 is transmitted to concrete structure 70through apertured bearing plate 80 positioned therebetween for loaddistribution purposes. If desired, cavity 74 can be filled by groutingafter completion of the post-tensioning and anchoring to form a smoothcontinuation of outer surface 72.

The anchoring system shown in FIG. 23 can be achieved, for example, byelongating the protruding end of tendon 4 by conventional hydraulicjacking procedures with the protruding end having been previouslyinserted through sheaths 90 and 92, with the sheath 92 having clampingnuts 94 in threaded engagement with apertures in the sheath wall, asshown in FIG. 24. After elongation of the wire, the clamping nuts 94 arethreaded into and against the tendon 4 to engage it with high frictionand to press it into corresponding recesses 93 in sheath 92. When thejacking force is removed (FIG. 25), the clamping action of nuts 94maintain the tendon in the elongated condition and leaves the protrudingend of the tendon 4 free to move with the crimping of the sheath 90 asshown in FIG. 26. Upon completion of this crimping, the clamping nuts 94can be unthreaded from the sheath 92, the sheath 92 slid over the freeend of the tendon 4 and the tendon trimmed back to the outer extremityof the crimped sheath 90, which would then resemble the crimped sheath78 of FIG. 23, confining the remaining protruding end of tendon 4 in thecrimped condition and transmitting a compressive force to the concretestructure 70 through apertured bearing plate 80.

The deformations and load carrying capacity in the crimped portion ofthe tendon are distributed along the length of crimping instead of beinglocalized. The stepwise increase in severity of the vending in thedirection away from the concrete structure leads to each bend in thetendon 4 carrying a portion of the tensile load of the post-tensionedtendon. This is a positive end anchor that depends on the deformation ofthe tendon and sheath rather than frictional engagement between thetendon and sheath. While each deformation of the tendon is a stressriser, the progressive increase in bend severity and thus stress risermoving away from the concrete is inverse to the variation in tensileload along the crimped portion of the tendon. The highest tensile loadis at the bend closest to the concrete structure, but that bend is theleast severe and therefore the lowest stress riser. The outermost bendis the highest stress riser, but the tensile load at that point is thelowest, increments of the tensile load having been assumed by precedingbends.

The number of reversals required for the crimped portion of the tendonand the crimped sheaths will depend on the tensile stress in the tendonand on the sharpness with which the tendon and sheath are bent.Generally, it is desirable to have the bend of each angle of the crimpprogressively decrease by at least about 5 moving in the direction awayfrom the concrete structure. Bend angle" means the included angle formedby the crimped tendon.

Equipment which is suitable for simultaneously forming all of the crimpsin the tendon end and sheath is shown in FIGS. 27 to 31. The apparatuscomprises a support structure 102 for positioning against the side of aconcrete structure 104 from which the end of a post-tensionable tendonprotrudes. In the embodiment of FIG. 27, the tendon is the compositetendon 14 of FIG. 6 but consisting of four tendons 106, with the tendons106 being fanned-out within the concrete structure,

so as to individually protrude from the side thereof. A sheath 108 isslipped over an end of one of the tendons 106 and the tendon ispost-tensioned using jaws 110 of a jack (not shown). The apparatus 100is provided with a fixed holding structure or jaw 112 for a series ofcrimping elements 1 14 on one side of the sheath 108 and on the otherside of the sheath, a movable holding structure or jaw 116 for crimpingelements 118. The crimping elements 114 and 118 are arranged such thatwhen jaw 116 moves towards jaw 112, the sheath is crimped into a seriesof reversals rather than pinched. This movement'is accomplished by ajack 1'20 mounted to the support structure 102 and operating on jaw 116.This operation is repeated for each tendon 106; as shown in FIG. 27, theoperation has been completed for one tendon 106. As shown in FIG. 28,the ends of the tendons 106 protrude in a staggered pattern from theconcrete structure 104 to enable the apparatus 100 to fit between thetendons.

The change in bend angle along the length of the crimp is accomplishedby the configuration of the surfaces of the crimping elements contactingthe sheath. FIG. 29 shows a representative series of crimping elements122, each of which have converging faces 124 terminating in an edge 126directed towards the longitudinal axis of sheath 108 containing tendon106 to be crimped. This series is useful in opposing relationship likeelements 114 and 118. The angles between the converging faces of eachcrimping element 122, viz., tooth angle, increases step-wise from rightto left from 60 to 160 as indicated, with the largest tooth angle beingclosest to the concrete structure. The larger the tooth angle, the lesssharp is the bend caused in the sheath. Generally, the bend angle in thesheath will correspond to the tooth angle.

As shown in FIGS. 30 and 31, the faces 124 and edge 126 of each crimpingelement 122 have a concavity 128 therein, forming a rounded forward edge130 where element comes into contact with the sheath 108.

All of the protruding ends of tendons 106 can be contained within asingle cavity formed by the concrete structure, such as cavity 74 ofFIG. 23, or separate such cavities can be provided for each tendon end.The relative sizes of the crimping apparatus and the cavity will controlwhether the former can be used within the cavity. If the crimpingapparatus does not fit within the cavity, the apparatus can be used inabutting relationship with the outer surface of the concrete structure,e.g., surface 72 of FIG. 23, with the crimped sheath-tendon beingallowed to withdraw into the cavity of the concrete structure, followedby grouting. While this withdrawal somewhat relieves the tensile forceof the tendon, such relief would be insignificant for long tendonlengths and moreover can be adequately compensated for by acorresponding increase in the tensile stress applied by post-tensioning.

While the foregoing method and apparatus for anchoring the end ofpost-tensioned tendons has been described for tendons 106 which are thesame as the tendon of FIG. 3, this method and apparatus can be used forpost-tensionable tendons in general, so long as the tendon is crimpablein the series of reversals described. The heavier the gauge of thetendon, the heavier is the crimping apparatus required. Thus, use ofl'll tendons of the present invention, e.g., of FIG. 3, is preferred,since the corrosion protection provided by the ingredients of layer 12and coating 10 of plastic material enable light gauge tendons to beemployed.

The tubular metallic sheath that is useful in the anchoring system ofthis invention can be of any metal and dimension which is sufficientlydeformable to undergo crimping without fracture and which retains thecrimp. Preferably, the sheath is corrosion resistant so as to protectthe crimped portion of the tendon from corrosion. Annealed stainlesssteel tubing, e.g., No. 304 or 410 stainless steel, is preferred. By wayof example, a crimping apparatus similar to that of FIG. 27, containingopposing series of crimping elements of FIG. 29 is used to crimp a No.410 stainless steel tubing inches long 0.1875 inch O.D. X 0.125 I.D.about an end of 0.080 inch diameter high tensile wire extending througha concrete slab and under a tension of 850 lb. The crimp is accomplishedusing a crimping force on the movable jaw of the apparatus of 8,960 lb.After crimping, the sheath exhibits bend angles closely approximatethose of the crimping elements. The tensile force remaining by the wireafter crimping is about 835 lb.

With the crimped sheath encasing the protruding end of the tendon, theanchoring system of this invention lends itself to complete protection,independent of grouting, of the protruding end of the tendon fromcorrosion and failure resulting from such corrosion. Such completeprotection can be obtained by providing sealing means at each end of thecrimped sheath for sealing the junction between the sheath and theconcrete and any open space between the ends of the sheath and thetendon encased thereby. In one embodiment, as shown in FIG. 32, part ofthis scaling is obtained by providing a waterproof gasket 132 compressedbetween a flared section 134 of a crimped sheath 136 and apost-tensioned tendon, e.g., tendon 4, extending into a cavity 138 in aconcrete structure 140. The sheath 136 resembles sheath 90 (FIG. 24)prior to crimping, except for the presence of the flared section 134which is preformed thereon. The angle which the flared section makeswith the precrimped longitudinal axis of the sheath is such that theflared section does not flare further to any appreciable amount when thetensile load of the wire is placed on the sheath. Generally, the flareangle should be between about and 30.

The gasket 132 has a circumferential groove 142 intermediate its endsfor retaining itself in an aperture in bearing plate 144 and a passage146 extending therethrough to permit the tendon to pass through thegasket. The outwardly directed section 148 of the gasket has a taperedsurface for mating with the inner surface of the flared section 134 ofthe sheath. The volume of the outer section 148 is sufficiently large,however, that the flared section 134 compresses it against the bearingplate and the tendon when the sheath 136 assumes the tensile load of thetendon, thereby preventing the intrusion of corrosive elements, e.g.,moisture. The gasket sealing arrangement can be used with tendons onwhich no plastic coating is present. The particular configuration of thegasket is unimportant so long as it is one which is compressed by theflared sheath to seal the junctions indicated. Suitable gasketingmaterials include rubber, natural and synthetic and plastics.

The extremity or outer end of the crimped sheath 136 can be madeself-sealing by deforming the sheath into the tendon. For example, asshown in FIG. 33, the end of the sheath 136 is uniformly and radiallydeformed into the tendon 4 slightly deforming it, whereby the plasticcoating on tendon 4 fills all space between the sheath and the wire ofthe tendon. The cavity 138 can also be filled with a waterproofingmaterial such as grouting.

Another sealing arrangement for use with the crimping technique foranchoring the ends of tendons according to the present invention isshown in FIG. 34 and 35, wherein a tendon, e.g., tendon 4, extendsthrough an aperture in a gasket 150 which is locked in place within anaperture in bearing plate 144 in concrete structure 140. A cuplikemember 152 having an aperture in its bottom through which tendon 4passes is positioned adjacent the bearing plate, followed by a sealingbushing 154, and a sheath 90. The tendon 4 is then tensioned and thesheath and crimped such as in the form shown in FIG. 23. Upon release ofthe tensioning force, the tendon withdraws slightly into the cuplikemember 152, causing the crimped sheath to compress the bushing 154,which, in turn flows to fill the space between the aperture in thecuplike member and the tendon and the spaces between the tendon andsheath and sheath and cuplike member, as shown in FIG. 35. This achievesa moistureproof seal of the tendon and the gasket 150, a cuplike member152, and sheath 90. The bushing 154 is made of moistureproof materialwhich is deformable under the conditions described. When the tendon isplastic coated, as is tendon 4 in the embodiment discussed, the materialfrom which bushing 154 is made is preferably softer than the plasticcoating on the tendon so as not to impair the integrity of the coatingor of the layer 12 underneath. Thus, for example, when the coating isnylon, the bushing can be polyethylene, and when the coating ispolyethylene, the bushing can be a softer polyethylene, e.g., one oflower molecular weight.

Although the anchoring systems discussed herein in detail have beenillustrated at one end of tendons only, these systems are equallyapplicable to the opposite ends or to other means for end fastening atsuch opposite ends.

BONDED TENDON Another embodiment of the present invention resides in theprovision of a post-tensionable tendon which is applied in the manner ofother post-tensionable tendons except that only temporary anchoring ofthe ends of the tendon are required to retain the concrete structureunder compression. This embodiment of tendon, as shown in FIG. 36,consists of an outer covering 160, a core 162, which is thepost-tensionable part of the tendon, and a layer 164 of a lubricous,curable bonding material, the covering and layer 164 being present alongthe length of the core. The material of layer 164 is lubricous to permitthe core to be post-tensioned while the covering 160 remains embedded inthe concrete structure and is curable to bond the tensioned core to thecovering and thus to the concrete structure along the length of thecore. The tensioning force on the core is then released. Because of thebond of the core to the concrete structure, the core remains elongatedand the concrete structure remains under compression.

The covering 160 can be the same as coating 10 previously described,with the tightness thereof being desired in this embodiment forachieving the bond uniformly along the length of the core. Theuniformity in the coating thickness and interior surface desired fortendon 4 is not as necessary for the embodiment of FIG. 36, however,unless low coefficients of friction are desired. Thus, in someapplications in which some sacrifice in coefficient of friction can betolerated, covering 160 can be of tape wrapped such as in helicalfashion about the layer 164 on core 162.

The core 162 can be the same as wire 8 of the tendon of FIG. 3, of anydiameter desired, or can be a multiple strand tendon.

The material of layer 164 can be one which is curable in situ to changefrom a liquid to a solid which bonds the core to the covering. Thematerial of layer 164 should not cure at room temperature so as to bestorage stable, but is heat activatable so as to be cured when desiredby heating. Typically, curable plastic materials (resins) can be used.For example, epoxy resins can be cured from the liquid to the solid formby heating in the presence of a curing (hardening) agent. Thus, thelayer 164 can be made of a blend of epoxy resin and curing agent whichdo not interact until some predetermined temperature is reached, e.g.,I00 to 18020 C. This temperature can be reached by connecting thepost-tensioned core to form a resistance portion of an electricalcircuit, to cause the wire to heat, thereby heating the epoxyresin-curing agent blend, causing it to cure and bond the core in theelongated condition. Solid lubricants can be present in the blend to aidin the uniform postitensioning of the core, but not to interfere withthe subsequent curing process.

Suitable epoxy resins include the hydrocarbon polyepoxides, i.e.,epoxides having more than one epoxy group per molecule. These includeepoxides containing ether linkages and both symmetrical and asymmetricalepoxides. Also included are the epoxides (commonly available as Epon"resins) prepared by reacting a polyhydric phenol with a polyfunctionalhalo-epoxy alkane. Curing agents include polyfunctional amines (commonlyavailable as Epon" curing agents), acids and acid anhydrides. Examplesof suitable epoxy resins and some high temperature curing agents aredisclosed in U.S. Pat. Nos. 2,500,600; 2,695,894 and 2,829,124. Specificepoxy resins include vinyl cyclohexane diepoxide (Unox 206), dipentenedioxide (Unox 269), and

(Epon 828) ADDITIONAL SPECIFIC EMBODIMENTS EXAMPLE 1 Detailsillustrating one application of one embodiment of low friction tendon ofthis invention are as follows: a form for the pouring of a concrete slabmeasuring 120 in. X 12 in. X 3 in. is prepared having 3 in. angle ironextending along each 12 in. dimension. The angle iron has apertures atin. spacing extending along a line which is 1% in. from the bottomsurface of the form. Nineteen tendons are laid out in separate straightand parallel paths within and along the length of the form, with theends of the tendons protruding through the apertures in the angle ironand tensioned with 100 lb. per tendon. Each tendon consists of a colddrawn carbon steel wire of 0.08 inch in diameter and having an elasticlimit of 240,000 p.s.i., a uniform layer of about 0.0007 inch inthickness of the corrosion inhibitor-lubricant hereinbefore described,blended with (50:50 mixture) a polytetrafluoroethylene dispersioncontaining 60 percent by weight solids, and a tight and uniform coatingof 610 nylon of 0.01 inch in thickness applied by a tubing typeextrusion die. The coefficient of friction between the wire and thecoating is 008. Concrete which has a compressive strength of about 5,000p.s.i. upon curing is poured into the form and allowed to harden. Metalsleeves measuring .0125 ID. are then positioned about each protrudingend of each tendon; each sleeve has four setscrews threaded thereinalong its length. The setscrews of the sleeves extending along one endof the slab are all tightened against the corresponding protruding endsof the tendons. The opposite protruding ends are individually jacked(jack grips bare wire of tendon) to a tensile reading of 1,000 lb.,thereby imposing a stress of 200,000 p.s.i. on each wire. While thestress is maintained the metal sleeve is moved to abut the angle iron,and the setscrews are tightened against the tendon to maintain thestress (except for diminishment due to creep) upon release of thejacking force. To test the load carrying capacity of the slab, it issupported from its ends by 4 inch X4 inch X4 inch blocks and loaded atthe center. The slab resting on the blocks supports a load of 425 lbs.prior to initial cracking and 750 lbs. before failure of the concrete.

EXAMPLE 2 In anotherapplication, a form for a slab measuring 32 inch X64 inch X 1V2 inch is constructed and two tendons are disposedtherewithin to form the tendon pattern shown in FIG. 17 in a plane aboutmidway in the 1% inch direction. Each tendon consists of a cold drawncarbon steel wire of 0.050 ,inch in diameter having an elastic limit of300,000 p.s.i., a uniform layer of the corrosioninhibitor-lubricant/polytetrafluoro ethylene dispersion blendhereinbefore described measuring about 0.005 inch in thickness, and atight, uniform coating of 610 nylon of 0.010 inch in thickness. Thereversals (totaling 450 for each tendon) in the tendon pattern areformed by passing the tendons around six inch diameter tubes. Concretewhich has a compressive strength of about 5,000 p.s.i. is poured intothe form and allowed to harden. The tendons are tensioned and clamped asdescribed in the preceding paragraph, with the jacking force of 470 lb.at one end of each tendon producing a force to about 400 lb. at each ofthe opposite ends thereof. These two tendons add 117 lb./ft. to theuniform load carrying capacity of the concrete slab. When the number oflike post-tensioned tendons in increased to 20 in a pattern like thatshown in FIG. 15, except that the tendons return to the starting end ofthe slab as shown in FIG. 17, the uniform load carrying capacity of theslab is increased by 11.7 lb./ft."-

EXAMPLE 3 These examples are repeated using polyethylene in one instanceand ethylene/methacrylic acid copolymer partially neutralized with metalion to form ionic copolymer such as described in U.S. Pat. No. 3,264,272to Rees, in another instance, in place of the nylon coating, to obtainsubstantially the same results.

EXAMPLE 4 In still another example, the low coefficient of frictionbetween wire and coating of plastic material was obtained by applying a2 mil coating of polytetrafluoroethylene enamel to the wire, followed bybaking, coating with corrosion inhibitorlubricant, and coating withplastic material.

EXAMPLE 4 (Bonded Tendon) A high-tensile steel wire 0.105 inch indiameter and having a breaking strength of approximately 2,500 lb.corresponding to a tensile strength of 300,000 p.s.i. was coated with aviscous layer of blend of Epon 828 epoxy resin containing a curing agentactive at temperatures above 100 C. The viscous epoxy and activator wasformulated in the following manner: The blend is prepared by mixingtogether 70 parts by weight of the epoxy resin with30 parts by weight(solids bases) of the curing agent which is a benzoguanamineformaldehyde normal butanol condensation product prepared under slightlyacid con ditions (66 percent solids by weight). This blend was milledtogether with 25 percent by weight of finely groundpolytetrafluoroethylene solid lubricant resin (granular) to form ahomogeneous mixture. The resultant blend was then applied to the wire toform a layer averaging 0.003 inch in thickness surrounding the wire. Thecoated wire was then dried and was helically wrapped with threeoverlapping layers of 1 inch wide ethylene/ vinyl acetate copolymercoated polyester film having a total thickness of 0.0015 inch. Thiswrapped tendon was then heated sufficiently at C. To melt theethylene/vinyl acetate copolymer coating on the polyester film to bondone layer of the film to the next, thereby creating an imperviouspolymer jacket.

The resultant tendon was positioned across a wooden concrete from of thetype used for reinforced concrete construction. Concrete was poured intothe form and cured, followed by jacking of the ends of the wire toproduce a desired axial stress in the wire of 2,000 pounds. An AC-DCwelding machine of a conventional type was connected to the wire and thewelding machine was energized to cause low voltage DC electrical powerto flow through the wire causing electrical resistance heating of thewire. Prior to pouring of the concrete thermocouple leads were connectedto the wire at a point within the wooden form. The amperage delivered tothe wire by the welding machine was adjusted to give a thermocouplereading indicating a temperature of 170 C. which was held for 60minutes. After allowing three hours for cooling, the jacks weredisconnected from the ends of the wire. No noticeable drawback of thewire into the concrete occurred. The 2,000 pound axial force in the wirewas transmitted to the concrete structure by the bond of the cured epoxyresin between the film wrapped covering and elongated wire. Todemonstrate the bond, one end of the wire extending from a 40 inch longsection of the concrete was jacked to produce an axial stress slightlyless than the breaking strength of the wire; the wire could not beextracted.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claims.

I claim:

1. A process for anchoring an end of a post-tensioning tendon whichprotrudes from a concrete structure so as to place said structure undercompression, comprising encasing said end of said tendon in a sheath,tensioning said tendon and crimping said sheath and said tensionedtendon encased thereby in a series of reversals of increasing angularityin the direction away from said concrete structure sufficiently tomaintain said tendon in the tensioned condition upon release of saidtensioning.

2. In the process of claim 1 placing a bearing plate between the sheathand the concrete structure to distribute the compressive force.

3. In the process of claim 1 using a tendon which is made of steel wirehaving an elastic limit of at least 150,000 p.s.i. and using a sheathwhich is a tubing of annealed stainless steel.

4. [n the process of claim 1 using a tendon which is made of cold drawncarbon steel wire having an elastic limit of at least 150,000 p.s.i. andusing a sheath which is a tubing of annealed stainless steel No. 410.

5. In the process of claim 1 using a tendon which is made by tightly anduniformly coating 21 steel wire having an elastic limit of at least150,000 p.s.i. with a plastic material, there being a coefficient offriction no greater than 0.09 between the wire and the coating, andusing a sheath which is a tubing of annealed stainless steel.

1. A process for anchoring an end of a post-tensioning tendon whichprotrudes from a concrete structure so as to place said structure undercompression, comprising encasing said end of said tendon in a sheath,tensioning said tendon and crimping said sheath and said tensionedtendon encased thereby in a series of reversals of increasing angularityin the direction away from said concrete structure sufficiently tomaintain said tendon in the tensioned condition upon release of saidtensioning.
 2. In the process of claim 1 placing a bearing plate betweenthe sheath and the concrete structure to distribute the compressiveforce.
 3. In the process of claim 1 using a tendon which is made ofsteel wire having an elastic limit of at least 150,000 p.s.i. and usinga sheath which is a tubing of annealed stainless steel.
 4. In theprocess of claim 1 using a tendon which is made of cold drawn carbonsteel wire having an elastic limit of at least 150,000 p.s.i. and usinga sheath which is a tubing of annealed stainless steel No.
 410. 5. Inthe process of claim 1 using a tendon which is made by tightly anduniformly coating a steel wire having an elastic limit of at least150,000 p.s.i. with a plastic material, there being a coefficient offriction no greater than 0.09 between the wire and the coating, andusing a sheath which is a tubing of annealed stainless steel.